Some Democratic lawmakers who support closing the prison at Guantanamo Bay, Cuba, say the United States should reconsider whether to repatriate suspected terrorists from Yemen, given the al Qaeda activity in the poor Arab nation.

President Obama’s top counterterrorism adviser, John Brennan, said Sunday the transfers will continue if the administration deems them warranted.

Six Yemenis who returned last month were released after the government there determined they were not a threat, officials in Yemen told the Associated Press.

Mr. Obama has said an al Qaeda group operating in Yemen apparently was behind the plot to bring down a U.S.-bound airliner on Christmas Day. The United States and Britain closed their embassies in Yemen on Sunday in response to threats from al Qaeda.

Although Republicans have criticized the transfers to Yemen, some Democrats, including Sen. Dianne Feinstein of California, the Intelligence Committee chairman, also have urged a halt.

Rep. Jane Harman, California Democrat and a member of the Homeland Security Committee, said Sunday that officials should review the transfers. She does support plans to close the prison and open one in Illinois for terrorism suspects.

“I think it is a bad time to send the 90 or so Yemenis back to Yemen,” Mrs. Harman said.

Sen. Joseph I. Lieberman, Connecticut independent, who has opposed closing Guantanamo, said transferring any of the Yemeni detainees back home would be irresponsible.

“We know from past experience that some of them will be back in the fight against us,” Mr. Lieberman said.

U.S. officials believe two Saudis released from Guantanamo, one in 2006 and the other in 2007, may have played significant roles in al Qaeda activities in Yemen.

An estimated 90 Yemenis are being held at Guantanamo Bay and about half are set to be sent to Yemen. Those who remain in U.S. custody will be prosecuted in criminal or military courts, Mr. Brennan said.

“Some of these individuals are going to be transferred back to Yemen at the right time and the right pace and in the right way,” Mr. Brennan said. “We’re making sure that the situation on the ground is taken into account, that we continue to work with the Yemeni government, and we do this in a very common-sense fashion because we want to make sure that we are able to close Guantanamo.”

Yemen has freed the six Yemenis who were released from Guantanamo Bay and returned to the country on Dec. 20, security officials and a lawyer for the men told the Associated Press.

The lawyer, Ahmed al-Arman, said the six were freed from Yemeni custody during the last week, with the last two freed Saturday night. They were handed over to their families.

Security officials held the six for questioning and investigation since their handover by the United States, but they found no evidence of involvement in terrorism or other crimes, Yemeni security officials said. The six gave guarantees that they would not leave the country, would not associate with terror groups and would report regularly to the police, the officials said, speaking on condition of anonymity because they were not authorized to talk to reporters.